When printing from computers or when copying entire digitalized pages with high resolution on so called page printers, there are created latent invisible electrostatically charged dots on a surface intended for the purpose, which dots together form a pattern, which shall correspond to the text or image intended to be printed.
During the subsequent step of the process this surface with its electrostatic screen pattern commonly is conveyed in front of adjacent charged particles, e.g. toner. By causing a sufficient potential difference between the screen dots, which shall remain non-blackened and the screen dots intended to be blackened by toner, it is effected that the charged particles jump over from a conveyor device, hereinafter referred to as the developer, to the surface charged in screen shape and form desired pattern. This part of the process hereinafter is named development.
This method earlier has been realized by using the technique now current in copying apparatuses--xerography, or similar variants of this process. Common for most page printers available on the market today is that they use an intermediate storing medium, often in form of a conductive roller, which is charged to a desired charge pattern, coated with carbon powder and which is finally brought to give off a carbon pattern to a paper or the like.
The most common method hereby is to use a photo-conductice roller, which is designed as a light sensitive surface layer, e.g. amorphous selenium or amorphous silicon. This roller is exposed dot-by-dot, often with monochromatic light, e.g. from a laser, as it rotates in front of the shutter of the light source. Another less frequent method is to deposit ions from a device down onto a drum coated with a surface layer suitable for the purpose.
Further another commercially unusual method is to use a particular paper coated with a conductive surface layer, e.g. zinc oxide, and to allow this to constitute the intermediary layer for the latent electrostatic image. The paper hereby passes a matrix of electrodes arranged orthogonally to the plane of the paper, which electrodes charge the surface layer of the paper to the desired screen image.
Common to all hitherto used methods are the high complexity of the equipment, a time consuming process and high service and maintenance requirements. A typical demand for resolution on the market today is 300 dots per inch. This demand for performance puts high requirements on tolerance and optical performance. Due to the short life span of conductive coatings and the comprehensive mechanism required for creating a xerographic process the above decribed methods result in high investment and operation costs for the user.
This becomes still more stressed for printers with high speed and resolution performances, where the requirements on the charging process of the drum increases the costs for the manufacturers. The method to use intermediate storing medium, in form of a conductive drum, for the electrostatic charges, also implies that a certain amount of toner will stick to the drum after the transfer to the paper was intended to take place. Such a device thus must also incorporate equipment for cleaning the drum after every single printing operation. This means more components and increased contamination with residual toner.
In order finally to create a good and permanent attraction power between the transferred particles and the paper, the paper usually passes a heating press intended for the purpose and consisting of two heated rollers being capable to melt the plastic layer on the particles. This equipment of course also increases the cost for the manufacture and reduces the accessibility of the machine.
The xerographic process furthermore involves a number of limitations regarding the quality of the print. Such a limitation is constituted by the unability of the intermediate storing medium to store high potential differences between white and black areas in a surface with a lower degree of blackening and a lower focusing as result. Another limitation is constituted by difficulties to control the individual size of the screen dots. This property causes inconvenience at reproduction of so called half-tone originals, where the size of every seprate screen dot represents a certain monochrome scale. For this purpose it has hereby been necessary to reserve a suitable number of adjacent screen dots at the printer for every new separate screen dot in the half-tone image. In this manner it is thereby possible to activate a suitable number of the printer screen dots for the purpose of varying the visual impression of the size of the screen dots of the half-tone image. This method reduces the resolution of the half-tone image as compared to the original performance of the printer.
It is earlier known, e.g. from U.S. Pat. No. 4,338,615 (Nelson, et al.), that many of those shortcomings are entirely or partly eliminated by letting a number of so called needle electrodes, which are orthogonal to the information carrier, be in electrostatic cooperation with the development process. As earlier known devices use needle electrodes, which can be individually activated, these always are arranged in one row or in a small number of rows, which rows often are of the same length as the width of the paper web, which is movable relative to the rows of needle electrodes which can be activated individually and are grouped in matrices on heads that are movable relative to the paper web. These methods incorporate a number of controllable screen dots for the entire printed page. As the electrostatic forces act upon a surface which during every moment of the development process, is bigger than the overall electrode size at these methods, the methods also must rely upon a conductive storing possibility for the adjacent screen dots, which risk to be blackened, as they are not in electrostatic cooperation with the electrodes. These methods therefore can not quite solve the above problems.
It furthermore has been established that non-permanent data representation from computers via viewing screens causes the operator inconveniences such as impaired readability and in certain cases radiation problems. Due to the requirements for speed in this information exchange the task has earlier been solved with aid of electron beam tubes, liquid crystals or plasma screens. A common characteristic for these methods is however the reduced readability.